The present invention relates to electronic line shafts in which servo driven motors simulate a mechanically continuous shaft and, in particular, to an electronic system for improving electronic line shaft operation.
Many industrial processes require a mechanical line shaft with intermediate gear boxes or the like to maintain precise phase or position among a number of driven structures. For example, in a printing press, each color must be precisely overlaid on the previous color to hold registration and this may be done by a mechanical line shaft joining each of the print cylinders.
Over time, the mechanical line shaft may wear, causing phase shift among the drives. Mechanical shaft systems are also cumbersome and difficult to reconfigure.
For these reasons it is known to create an electronic line shaft in which a master reference is established, for example, one cylinder on a printing press, and its position and phase captured by an encoder. The encoder signals are forwarded to servomotors attached to the other print cylinders, each cylinder having a motor encoder feedback unit that matches the position of the cylinder to the master reference. Alternatively, the master reference may be a signal generator, generating a virtual encoder signal with all cylinders being locked to that signal.
While the electronic signals transmitted from the master reference or virtual master reference to each of the servant drives propagate at an extremely high rate, it is not instantaneous and thus there may be some phase lag at servant motor drives that are displaced spatially or electronically from the master reference. This problem is particularly acute for networks which provide simplicity in connecting motor drives, but which introduce additional network-type propagation delays.
Propagation delays in the transmission of the encoder signals among the various drive units may be accommodated somewhat by, for example, advancing the cylinders associated with the delayed units to offset the delay in their receipt of the signal. This approach of anticipating network delay, however, is not always successful because of a variation in the delay depending on network traffic and other factors. Further, for systems that work at different rotational speeds, advancing the cylinder positions, for example by adjusting the encoders or the like, for a given speed will produce phase errors for lower speeds where the propagation delay results in an effectively smaller phase advance.